This topic contrasts with my essay on the Source (that is, where magic itself comes from), in that this focuses on where the energy necessary to break the standard laws of physics come from.
There are three archetypes of power I will use to categorize systems: Free, Renewable, and Nonrenewable.
Free sources are most common in Inherent and Soft systems: magic is readily available and unlimited and requires no energy (or at least costs no energy) to cast. Harry Potter is, of course, a foremost example of this. All that is required is that the invocation is performed and the proper materials are assembled.
Renewable sources are common among Harder systems. In a renewable system, magic is freely reproducable, but can only be stored in certain quantities. Any kind of system with mana, such as Stormlight in The Stormlight Archive, or one’s own stamina in Eragon, falls under this category. A person can only hold so much magic at a time, but can refresh that quantity essentially for free if they ever run out.
Nonrenewable sources are very rare. I’ve only ever seen one system that fuctions this way: Sanderson’s Allomancy, in which small bits of metal are annihilated for special powers. Here, magic is a finite resource that will eventually run out (or at the power source will). In the world of Mistborn, where Allomancy exists, the nonrenewability of Allomantic magic is balanced by the fact only a few people are able to use it, and only need a very small amount of a given metal at a time.
Personally, I prefer renewable source systems, in that they present a problem of resource management, but one doesn’t have to worry about magic ever actually running out. However, all three have a myriad of uses and contexts to excell in, depending on the focus of the narrative.